The article's problem with "uncertain" is that it has connotations of some intelligence comprehending the observed value. When I say, "velocity is uncertain" it seems to mean something like "I am uncertain what the velocity is", but when I say that "velocity is undefined" or "velocity is fuzzy" then it is more clear that I am talking about the velocity itself.

I think "uncertain" is fine. Of course, the particular words are not really that important if you just tell people to read the definitions and equations used in the theory.